The invention relates to a process for coating electrically conductive substrates, in which
(1) the electrically conductive substrate is dipped into a water-based electro-dipping primer PA1 (2) the substrate is connected as the cathode PA1 (3) a film is deposited on the substrate by means of direct current PA1 (4) the coated substrate is removed from the electro-dipping primer and PA1 (5) the deposited film coating is baked. PA1 a) a diol and/or a triol or a mixture of diols and/or a mixture of triols, PA1 b) diphenylmethane 4,4'-diisocyanate the melting point of which has been depressed, by partial reaction with a monool and/or polyol or a mixture of monools and/or a mixture of polyols, so far that it is liquid at room temperature and PA1 c) a masking agent or a mixture of masking agents PA1 (.alpha.) optionally modified polyepoxides and PA1 (.beta.) amines. PA1 (i) a diepoxide compound or a mixure of diepoxide compounds having an epoxide equivalent weight of less than 2000 with PA1 (ii) a compound which, under the given reaction conditions, reacts in a monofunctional manner toward epoxide groups and contains a phenol or thiol group, or a mixture of such compounds, PA1 compounds containing carboxyl groups, such as saturated or unsaturated monocarboxylic acids (for example benzoic acid, linseed oil fatty acid, 2-ethylhexanoic acid and Versatic acid), aliphatic, cycloaliphatic and/or aromatic dicarboxylic acids of various chain lengths (for example adipic acid, sebacic acid, isophthalic acid or dimeric fatty acids), hydroxyalkylcarboxylic acids (for example lactic acid or dimethylolpropionic acid) and also polyesters containing carboxyl groups or PA1 compounds containing amino groups, such as diethylamine or ethylhexylamine, or diamines having secondary amino groups, for example N,N'-dialkylalkylenediamines, such as dimethylethylenediamine, N,N'-dialkylpolyoxyalkyleneamines, such as N,N'-dimethylpolyoxypropylenediamine, cyanoalkylated alkylenediamines, such as bis-N,N'-cyanoethylethylenediamine, cyanoalkylated polyoxyalkyleneamines, such as bis N,N'-cyanoethylpolyoxypropylenediamine, polyaminoamides, such as, for example, Versamides, in particular reaction products containing terminal amino groups and formed from diamines (for example hexamethylenediamine) and polycarboxylic acids, in particular dimeric fatty acids, and monocarboxylic acids, in particular fatty acids, or the reaction product of one mole of diaminohexane with two moles of a monoglycidyl ether or monoglycidyl ester, especially glycidyl esters of .alpha.-branched fatty acids, such as Versatic acid, or PA1 compounds containing hydroxyl groups, such as neopentyl glycol, bisethoxylated neopentyl glycol, neopentyl glycol hydroxypivalate, dimethylhydantoin-N,N'-diethanol, 1,6-hexanediol, 2,5-hexanediol, 1,4-bis-(hydroxymethyl)-cyclohexane, 1,1-isopropylidene-bis-(p-phenoxy)-2-propanol, trimethylolpropane or pentaerythritol, or aminoalcohols, such as triethanolamine or methyldiethanolamine, or alkylketimines containing hydroxyl groups, such as aminomethyl-1,3-propanediolmethyl isobutyl ketimine or tris-(hydroxymethyl)-aminomethanecyclohexanone ketimine, and also polyglycol ethers, polyester-polyols, polyether-polyols, polycaprolactone-polyols or polycaprolactam-polyols of various functionalities and molecular weights or PA1 saturated or unsaturated fatty acid methyl esters which are transesterified in the presence of sodium methylate with hydroxyl groups of the epoxide resins.
The invention also relates to water-based coatings and crosslinking agents containing masked NCO groups.
The cathodic electro-dipping process described above is known (cf., for example, German Offenlegungsschrift 3,518,732, German Offenlegungsschrift 3,518,770, EP-A 4090, EP-A 12,463 and EP-A 262,069) and is employed, in particular, for priming motor vehicle bodies, especially automobile bodies.
Coatings of excellent quality can be obtained by means of the cathodic electro-dipping process--particularly if electro-dipping primers containing cationic, amine-modified epoxide resins are used as binders.
The electro-dipping primers employed for carrying out the cathodic electro-dipping process preferably contain masked polyisocyanates as crosslinking agents. It is known that if masked diphenylmethane 4,4'-diisocyanate (MDI) is used as the crosslinking agent, the troublesome discoloration (yellowing) of topcoat layers which is observed when other aromatic polyisocyanates are used does not occur. MDI can, however, not be employed without difficulty, because it exhibits a strong tendency to crystallize and thus leads to the destabilization of the electro-dipping primers. It is stated in EP-A 236,050 that stable electro-dipping primers should be obtainable using MDI containing at least 5% by weight of the 2,4'-isomer. However, commercially obtainable MDI grades meeting this condition as a rule have, disadvantageously, a high content of hydrolyzable chlorine leading to electro-dipping primers of high chloride ion contents, i.e. of increased corrosivity. It is stated in EP-A 293,088 that stable electro-dipping primers should be obtainable using MDI containing a uretonimine. However, this suggestion for solving the crystallization problem unfortunately does not lead to success in all cases.